The present invention is directed, in general, to operational amplifiers and, more specifically, to a high current gain operational amplifier for driving low impedance wires in a local area network (LAN).
The rapid proliferation of local area network (LANs) in the corporate environment and the increased demand for time-sensitive delivery of messages and data between users has spurred development of high-speed (gigabit) Ethernet LANs. The 100BASE-TX Ethernet LANs using category-5 (CAT-5) copper wire and the 1000BASE-T Ethernet LANs capable of one gigabit per second (1 Gbps) data rates over CAT-5 data grade wire require new techniques for the transfer of high-speed data symbols.
A 1000BASE-T Ethernet LAN driver requires an operational amplifier (opamp) capable of driving low impedance loads (i.e., 50 ohm transmission lines) with a large signal swing and high linearity. In order to reduce high frequency energy, the data signal must also be low-pass filtered to 90 MHz. In conventional operational amplifier applications, this means that the opamp used in the filter must have a unity gain frequency which is much higher than 90 MHz. In order to achieve such a high unity gain bandwidth, a conventional opamp operates using a large amount of current (i.e., high power consumption). The high unity gain frequency also makes it difficult to stablize the operational amplifier and it usually suffers from poor phase margins. The operational amplifier also undergoes a degradation in the unity gain bandwidth due to the presence of an input pole in the feedback path.
Therefore, there is a need in the art for an improved operational amplifier that consumes less current when driving a low impedance transmission line. In particular, there is a need in the art for an operational amplifier that does not require a unity gain bandwidth that is much larger than the low-pass filtered frequency band of the transmission line. More particularly, there is a need in the art for an operational amplifier that eliminates the input pole in the feedback path.
The limitations inherent in the prior art described above are overcome by the present invention which provides an operational amplifier having a low impedance input and a high current gain output. According to an advantageous embodiment of the present invention, the operational amplifier comprises: 1) a first N-channel transistor having a source coupled to the low impedance input of the operational amplifier; 2) a first constant current source coupled between the source of the first N-channel transistor and ground; 3) a first amplifier stage having an input coupled to the first N-channel transistor source and an inverting output coupled to a gate of the first N-channel transistor; 4) a second amplifier stage having an input coupled to a drain of the first N-channel transistor and an output coupled to the high current gain output of the operational amplifier; and 5) an internal compensation capacitor coupled between the input and the output of the second amplifier stage.
According to one embodiment of the present invention, the operational amplifier further comprises a second constant current source coupled between the drain of the first N-channel transistor and a positive power supply.
According to another embodiment of the present invention, the first amplifier stage comprises a second N-channel transistor having a gate coupled to the source of the first N-channel transistor, a source coupled to ground, and a drain coupled to the inverting output of the first amplifier stage.
According to still another embodiment of the present invention, the drain of the second N-channel transistor is further coupled to a third constant current source.
According to yet another embodiment of the present invention, the second amplifier stage comprises a third N-channel transistor having a gate coupled to the input of second amplifier stage, a source coupled to a fourth constant current source, and a drain coupled to the positive power supply.
According to a further embodiment of the present invention, the second amplifier stage comprises a fourth N-channel transistor having a gate coupled to the source of the third N-channel transistor, a source coupled to ground, and a drain coupled to the output of the second amplifier stage.
The foregoing has outlined rather broadly the features and technical advantages of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features and advantages of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they may readily use the conception and the specific embodiment disclosed as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.
Before undertaking the DETAILED DESCRIPTION OF THE INVENTION, it may be advantageous to set forth definitions of certain words and phrases used throughout this patent document: the terms xe2x80x9cincludexe2x80x9d and xe2x80x9ccomprise,xe2x80x9d as well as derivatives thereof, mean inclusion without limitation; the term xe2x80x9cor,xe2x80x9d is inclusive, meaning and/or; the phrases xe2x80x9cassociated withxe2x80x9d and xe2x80x9cassociated therewith,xe2x80x9d as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like; and the term xe2x80x9ccontrollerxe2x80x9d means any device, system or part thereof that controls at least one operation, such a device may be implemented in hardware, firmware or software, or some combination of at least two of the same. It should be noted that the functionality associated with any particular controller may be centralized or distributed, whether locally or remotely. Definitions for certain words and phrases are provided throughout this patent document, those of ordinary skill in the art should understand that in many, if not most instances, such definitions apply to prior, as well as future uses of such defined words and phrases.